1. Technical Field
The present invention relates to a sensing wire-type temperature control device and a safety device for the sensing wire-type temperature control device, in connection with a heating cable, which is commonly used in a heating apparatus, e.g., an electric heater such as an electric blanket and an electric mattress pad, controlling the temperature by sensing a change in temperature by use of a sensing wire and being insulated with a thermo-sensitive resin that changes its impedance in accordance with the change in temperature between an electric heating body and the sensing wire.
The present invention also relates to a temperature control device using a thermo-sensitive insulation resin of an electric heater that can reduce a magnetic field generated by an electric heating body, through which electric currents flow in opposite directions.
2. Description of the Related Art
To control the temperature of an electric heater, two methods are commonly used. One is a thermometer method in which a temperature sensor is installed in the electric heater to control the temperature of the electric heater, which is heated by an electric current flowing through its heating wire, so that the change in temperature can be detected and controlled. The other is a sensing wire method in which a thermo-sensitive insulation resin is used to detect and control the change in electric current flowing through its sensing wire by using the impedance of a nylon thermistor, which is an insulation covering.
As illustrated in FIG. 1, an electric heating cable according to the sensing wire method using thermo-sensitive insulation resin may include a heating wire insulated by a nylon thermistor (NTC), which changes its impedance in accordance with the temperature of the heating wire, a sensing wire, which is wound on the nylon thermistor, and an exterior insulation covering, which covers the sensing wire.
In the sensing wire method using the thermo-sensitive insulation resin, the heating wire produces heat when an electric current flows through the heating wire, and the sensing wire controls the temperature of the electric heating cable by sensing the change in electric current that is caused by the changing impedance of the thermo-sensitive resin, which is changed in accordance with the temperature of the heating wire.
Compared to the method of using a separate temperature sensor and a bi-metal, which is for preventing overheating, the sensing wire method using the thermo-sensitive insulation resin is widely used because it uses a typical heating wire and the thermosensitive wire itself can not only sense a change in temperature but also prevent overheating, thereby facilitating easier installation and lowering the manufacturing costs.
Furthermore, since the sensing wire is wound on the heating wire from one end to the other, its ability to prevent local overheating is much more reliable than the thermometer method using a typical temperature sensor.
For example, in the case of an electric mat, if the electric mat is folded or a heavy object, for example, a pillow, is placed on the electric mat, the thermometer method, which generally uses two temperature sensors and two bi-metals for a typical electric mat for two persons would not accurately sense the change in temperature if the portion that is partially folded or loaded with the heavy object is too far away from the temperature sensors and the bi-metals.
Consequently, the folded portion may be overheated compared to the unfolded portions because its temperature is not sensed by the temperature sensors.
Installing more temperature sensors in the electric mat may increase the reliability of sensing the overheating but would be practically impossible due to the working conditions or manufacturing costs.
Compared to the thermometer method, the sensing wire method using the thermo-sensitive insulation resin can sense the change in temperature even if a certain portion is folded or pressed, because the sensing wire is installed throughout the heating wire. Thus, overheating can be prevented from occurring despite a local overheating.
Nevertheless, the sensing wire method is unable to detect local overheating completely with the 100 percent accuracy. As a result, there have been complaints raised by the users every year due to accidents, for example, fires and burns, caused by the local overheating.
This is because the electric currents show different values for different overheating areas although the temperature rise is the same, due to different voltages caused by voltage drop at different locations of the sensing wire using a nylon thermistor.
Therefore, temperature may be detected differently at different locations of the heating wire, causing a burn to the user, causing a fire or shortening the product life.
Moreover, in case local overheating occurs at an area near the ground, sensing currents may not be sufficient enough to generate a signal to cut off the power supply despite the continuous increase in temperature. Consequently, the heating cable can reach dangerous temperatures to cause a burn or fire.
In the case of electric mats and electric floor mats, the heating wire is typically installed in the method illustrated in FIG. 2. However, if the temperature control device is unable to accurately detect an increase in temperature at different locations, as described above, the temperature may or may not be properly controlled depending on the location where the user lies and may cause overheating.
Even though the temperature control device is preset by the user at a desirable temperature, the temperature of the heating cable may be rise or drop depending on the location where the user lies. As a result, the user may have to turn the dial up or down to maintain the desirable temperature, causing inconvenience to the user.
Also, if a defect occurs in some parts of the temperature control device (especially if a power control component (SCR) malfunctions so that electrical conduction is formed, or if the change in temperature is detected inaccurately because of a short-circuit in the sensing wire), the accidents described above may occur.
Therefore, a minimal safety measure is inevitably needed.
Recently, as it has become known that a magnetic field may be harmful to humans, the development of a sensing wire that can block a harmful magnetic field is currently under way. KR Patent Publication No. 1999-012089 discloses a way of blocking a harmful magnetic field. In FIG. 1 of this example, two heating wires are combined to form a double structure, and a terminal unit is electrically connected to the double structure. When power is supplied to the heating wires, the electric currents flowing through the two parallel heating wires flow in opposite directions, and thus a magnetic field formed between the two heating wires can be offset by the two opposite flowing currents.
In this type of heating wires for blocking a magnetic field, the two heating wires may be closely positioned by interposing an insulation material in between them, or a PVC or silicon covering may be formed around one heating wire, and then another heating wire may be wound on the heating wire so as to block the magnetic field.
Although such methods described above may block the harmful magnetic field, a heating current still flows through the sensing wire so that the sensing wire may be unable to detect a minute change in electric current according to the change in impedance of the thermo-sensitive insulation resin.
Therefore, the sensing wire method using the thermo-sensitive insulation resin may not be used alone, and an additional temperature controlling method must be employed. As a result, the manufacturing costs, the manufacturing process and the manufacturing time may be increased.
The technology shown in FIG. 3 is a heating wire structure that can block a magnetic field, and was disclosed by the inventor of this application to complement those disadvantages described above in KR Patent No. 10-0871682.
The technology shown in FIG. 2 is a triple-structured electric heating cable, in which a first wire, i.e., the main wire, is used as a heating wire and then insulated with a nylon thermo-sensitive resin, i.e., the insulation covering. After the nylon thermo-sensitive resin is formed around the first wire, a second wire, i.e., the temperature sensing wire, is wound on the nylon thermo-sensitive resin.
Afterwards, a PVC covering is formed around the outer surface of the temperature sensing wire, and then a third wire, i.e., the heating wire, is wound on the PVC covering. Therefore, the first wire and the third wire are electrically connected to each other.
In this structure, when power is supplied to the first wire and the third wire, the electric currents flowing in the first and third wires flow in opposite directions, and thus the magnetic field formed between them can be offset by the opposite flowing currents. Also, the temperature of the system can be controlled by measuring the change in impedance of the thermo-sensitive resin, which is interposed between the first wire and the second wire.
Although the method described above complements some problems associated with a method using an additional temperature sensor or bi-metal, the triple-structured electric heating cable becomes too thick to be employed in a thinner product, for example, a carpet or a blanket. Also, this method still does not reduce the manufacturing costs and shorten the manufacturing time.